The present invention is related to a device for sensing the position of a movable inertia body in a vehicle safety system in which the inertia body can be either in a neutral position,in which the body does not activate the safety system, or in at least one working position, in which the body activates the safety system. The device is primarily intended to be used in connection with standing, tiltable inertia bodies but can also be used in connection with movable inertia bodies of other types in vehicle safety systems. Such safety systems can comprise for example safety belts, inflatable air cushions and belt stretchers.
It is previously known to sense the position of movable inertia bodies used in locking devices for vehicle safety belts by means of displaceable bars or pivoting arms abutting against the inertia bodies and operated by the movements of the bodies. One example of such a device is disclosed in the British Patent Specification No. 1 351 642. One disadvantage of this type of sensing device is that the sensing means by its friction and mechanical inertia impedes the movements of the inertia body, so that the sensitivity of the locking device is influenced and can vary within rather wide limits. A particular disadvantage in connection with standing, tiltable inertia means intended to give the locking device the same sensitivity irrespective of the direction of the appearing force is that said sensitivity can be lost, because the sensing means may influence the inertia body differently in different moving directions of the inertia body.
Furthermore, through the German Patent Specification No. 26 30 553 it is previously known to use light rays or radiation from other radiating sources to sense or detect the position or the movements of inertia bodies in locking devices for vehicle safety belts. It is particularly previously known to use light rays that can be reflected by a surface of an inertia body, for example a pendulum, and detected by a light sensitive means. When the inertia body changes position, the output from the light sensitive means is changed, and this output is used for controlling the locking device. One disadvantage of this type of sensing device is that the light radiating means and the light detecting means must be positioned with a high degree of accuracy in relation to the reflecting surface of the inertia body to provide a reliable operation. This is particularly difficult to achieve, when the inertia body is a standing, tiltable inertia means.